In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of offspring and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is frequently used in sociobiology and evolutionary biology circles. The term often implies either a lifelong socially monogamous relationship or a stage of mating interaction in socially monogamous species. It is sometimes used in reference to human relationships.
Monogamous voles (such as prairie voles) have significantly greater density and distribution of vasopressin receptors in their brain when compared to polygamous voles. These differences are located in the ventral forebrain and the dopamine-mediated reward pathway.
Peptide arginine vasopressin (AVP), dopamine, and oxytocin act in this region to coordinate rewarding activities such as mating, and regulate selective affiliation. These species-specific differences have shown to correlate with social behaviors, and in monogamous prairie voles are important for facilitation of pair bonding. When compared to montane voles, which are polygamous, monogamous prairie voles appear to have more of these AVP and oxytocin neurotransmitter receptors. It is important that these receptors are in the reward centers of the brain because that could lead to a conditioned partner in the prairie vole compared to the montane vole which would explain why the prairie vole forms pair bonds and the montane vole does not.
Humans can experience some or all of the above-mentioned varieties of pair bonds in their lifetime. These bonds can be temporary or last a lifetime, same age or with different age groups. In a biological sense there are two main types of pair bonds exhibited in humans: social pair bonding and sexual pair bonding. The social pair bond is a strong behavioral and psychological relationship between two individuals that is measurably different in physiological and emotional terms from general friendships or other acquaintance relationships. On the other hand, the sexual pair bond is a behavioral and physiological bond between two individuals with a strong sexual attraction component. In this bond the participants in the sexual pair bond prefer to have sex with each other over other options. Social pair bonds are usually more wide-ranging than their sexual counterparts due to the sexual nature involved in the latter. In humans and other mammals, these pair bonds are created by a combination of social interaction and biological factors including neurotransmitters like oxytocin, vasopressin, and dopamine.
Pair bonds (social and/or sexual) are a biological phenomenon and are not equivalent to the human social institution of marriage. Marriage can be associated with a sexual or social pair bond; however, married couples do not necessarily have to experience both or either of these bonds. Marriage can be a consequence of pair bonding and vice versa; however, neither always creates or leads to the other. Pair bonding in humans helps explain extreme "bonds" that we may share with others but are unable to articulate in terms of contemporary "love".
Close to ninety percent of known avian species are monogamous, compared to five percent of known mammalian species. The majority of monogamous avians form long-term pair bonds which typically result in seasonal mating: these species breed with a single partner, raise their young, and then pair up with a new mate to repeat the cycle during the next season. Some avians such as swans, bald eagles, California condors, and the Atlantic Puffin are not only monogamous, but also form lifelong pair bonds.
For about four days immediately prior to egg-laying, when copulations lead to fertilizations, the male bank swallow is very busy, attentively guarding his female. Before this time, as well as after--that is, when her eggs are not ripe, and again after his genes are safely tucked away inside the shells--he goes seeking extra-pair copulations with the mates of other males...who, of course, are busy with defensive mate-guarding of their own.
In various species, males provide parental care and females mate with multiple males. For example, recent studies show that extra-pair copulation frequently occurs in monogamous birds in which a "social" father provides intensive care for its "social" offspring.
A University of Florida scientist reports that male sand gobies work harder at building nests and taking care of eggs when females are present - the first time such "courtship parental care" has been documented in any species.
In the cichlid species Tropheus moorii, a male and female will form a temporary monogamous pair bond and spawn; after which, the female leaves to mouthbrood the eggs on her own. T. moorii broods exhibit genetic monogamy (all eggs in a brood are fertilized by a single male). Another mouth brooding cichlid - the Lake Tanganyika cichlid (Xenotilapia rotundiventralis) has been shown that mating pairs maintain pair bonds at least until the shift of young from female to male.  More recently the Australian Murray cod has been seen maintaining pair bonds over 3 years 
As noted above, different species of voles vary in their sexual behavior, and these differences correlate with expression levels of vasopressin receptors in reward areas of the brain. Scientists were able to change adult male montane voles' behavior to resemble that of monogamous prairie voles in experiments in which vasopressin receptors were introduced into the brain of male montane voles.